ABSTRACT— The hepatotoxic effect of phalloidin, a mushroom poison, was examined in whole liver and in isolated liver cell plasma membranes. By electron microscopy, a thickening of the pericanalicular microfilamentous network was noted. Bile canalicular enriched plasma membrane fractions of livers from phalloidin-treated animals contained increased amounts of microfilaments. Analysis of the peptides in these fractions, by sodium dodecyl sulfate polyacrylamide gel electrophoresis, indicated that although the actin peptide bands were prominent, a band apparently corresponding to myosin was drastically reduced. The morphological change was accompanied by a reduced bile flow and biliary excretion of bile acids, proteins and the exogenous organic anion dibromo-sulphthalein. These changes may result from a modification of the microfilaments, suggesting a role of the microfilaments in the liver membrane structure and function.